You would spend days discussing it with them, deciding the functions you want in it, figuring out and drawing up prototypes, communicating back and forth, reviewing each mock-up, going at it stage by stage...
You see, I've done this before when I built a custom smartphone app for a client. It was cumbersome, slow, and expensive. And when it was all done, it still had to be reworked, which meant cost overruns...aggh.